For channel set-ups from a single group of subscriber lines, constituting a single incoming TDM line, to a single outgoing PCM link, a single time stage for switching in time is sufficient for providing a set-up between an arbitrary subscriber line and an arbitrary link channel. However, a large digital concentrator including a plurality of subscriber line groups and a plurality of PCM links going to a central transit exchange needs, in principle, both a time stage for switching in time and a space stage for switching from an arbitrary incoming TDM line to an arbitrary outgoing PCM link.
It is known, e.g. from the U.S. Pat. No. 4,167,652 that the space stage can be avoided if a TDM bus system is provided, to the input modules of which are connected all TDM lines for transferring of information coming from the subscribers. PCM links for transferring information intended for the central exchange, as well as outgoing TDM lines for transferring internal information from one to the other of the subscribers without loading the PCM links, may be connected to the output modules of the bus system.
A so-called time-time system is obtained, which carries out a first time switch from an incoming subscriber channel to a bus channel and a second time switch from this bus channel, either to an outgoing link channel or to an outgoing subscriber channel. The time-time system is congestion-free if the number of bus channels is just as great as the total number of subscriber lines, the concentrator effect then occurring between the bus and the outgoing PCM links. Known TDM buses in large time-time systems are very high-speed and include very complicated input and output modules. Designs with two-way traffic on the bus and loop bus designs for one-way traffic are also known.